1. Field of the Invention
The present invention relates generally to satellite communications earth station antenna mounts and in particular to a computer controlled antenna mount system and its application in determining and maintaining optimal tracking of geosynchronous satellites whose orbits incline with respect to the equator.
2. Description of the Related Art
Satellites in geosynchronous orbit travel at precisely the same rotational speed as the earth. Thus, a satellite in geosynchronous orbit appears stationary from any point on earth that is within the satellite horizon. In other words, the geosynchronous satellite does not rise or set on the horizon as does the sun and moon, rather, it appears to be continuously at a particular point in the sky.
Many satellites have been launched into geosynchronous orbits with each satellite having a slightly different east-west position in the sky. All of the satellites normally are within a narrow ring called the geosynchronous arc that is within some fixed angle of the south to north horizons at any given point on earth. A polar oriented antenna mount may track satellites in this geosynchronous arc by rotating the mount about the polar axis. Thus, a ground station antenna may be aligned on one geosynchronously orbiting satellite and then may be rotated about its polar axis for acquisition of other such satellites.
Polar mounts allow alignment of ground station antennas with different geosynchronous satellites by rotation about its (single) polar axis. This single-axis tracking is both simple and economical for geosynchronous satellites that do not vary their east-west position significantly. However, any one of these satellites may have appreciable position change in the north-south direction due to orbital inclination. Normally, any non-geosynchronous position change of a satellite requires an antenna mount with azimuth, elevation and polar control for three axis tracking.
Three axis tracking antenna mounts are more complicated and expensive than single axis (polar only) mounts. In addition, the angular position rate of change required to track a geosynchronous satellite in inclined orbit is very slow. Typically, pointing angle changes are from less than one tenth of one degree (0.1) to one degree per hour. For comparison the hour hand on a standard clock turns at the rate of 30 degrees per hour. This slow and minute position change requires precise and repeatable orientation of the antenna mount.
A geosynchronous satellite may change its inclination orbit over a period of time. Different satellites may have different inclination orbits. Thus, the mount holding the earth station antenna must be able to function for different satellite inclination orbits and the corresponding different positional rate of change requirements. A further problem is that the inclination in orbit may vary over the useful life of a satellite.